The invention concerns a photoelectric scanner for scanning a transparent master.
Scanners of this type make possible the automatic analysis of the color composition in particular of copy masters. A scanner of this type is known for example from U.S. Pat. No. 3,944,362. In the scanner described therein the copy master in the form of a film is exposed to light from a source of light, after the light has been scattered by a diffusor. Following its passage through the film, the light enters an optical fiber. The light exiting from the optical fibers at the other end impacts a rotating disk having an orifice. The orifice in the course of its rotation releases in succession the light exiting from the individual optical fibers, which is detected.
Another scanner is known from FR-OS-2 621 710. In the case of the scanner described therein part of the light coming from a source of light impacts an optical fiber after passing through a filter located in an orifice of a rotating filter wheel. Following its exit from the other end of the optical fiber, the light passes through a diffusor and impacts a copy master in the form of a film. After passing through the film, the light enters other optical fibers, which lead to photocells. Another part of the light from the source of light is guided to a photosensitive paper.
A further scanner is known from DE-OS-24 59 456. In the scanner described therein, the scanning light coming from a source of light is guided over the film (copy master) by reflection from the mirror surfaces of a motor driven mirror wheel transversely to the longitudinal direction of the film.
Another scanner is known from DE-OS-26 27 694. In the case of this scanner the light of the light source travels through a diffusor and impacts the copy master in the form of a film. The light passing through the film is received by optical fibers whose light incident surfaces are located transversely to the transport direction, over the film. The light emission surfaces, i.e., the other ends of the optical fibers, are placed on a circle. The optical fibers are faced by a rotating disk comprising at least three windows in which filters for the colors red, green and blue are located. The scanner contains photoelectric converters to receive the light exiting from the light emission surfaces and passing through the filters of the rotating disks. The scanner further has two measuring or detecting devices: a first device to determine the rotating position of the rotating disk, with the first detection device providing a signal whenever a filter of a predetermined color is located in front of the light emission surfaces, and a second device to issue a command for the measurement of density whenever the filter of any color is located in front of the light emission surfaces.
Scanners of this type are used for example in high capacity printers. In the case of another known scanner used in such applications a light source projects a beam of light onto a circular disk, which rotates at a short distance parallel to the copy master and contains orifices to define a point to be scanned on the copy master. The light coming from the copy master is received by a detection unit.
The trend in the scanning of copy masters tends toward higher and higher resolutions. This increasingly requires smaller and more numerous scanning points per copy master, which in turn and in different ways requires greater capacities of the detectors and of the evaluating electronics. On the one hand, the scanning time per point must be as short as possible, as the number of points in the case of higher resolutions is higher, and on the other, the minimally detectable light intensity must be lower, since with identical light intensities in the scanning point but with smaller dimensions of said point, the quantity of the light impacting the detector will be correspondingly smaller. In the aforementioned scanners the light is initially scattered by a diffusor and then impacts the copy master. If by subsequent measures the dimensions of the scanning points are reduced, a loss of light power always follows. This renders the detection of the scanning light considerably more difficult.